New structure

Changes:

* Support Rails 3.2 is discontinued. Supported all Rails 4.x versions.
* Using `setup` instead of `configure` in initializer for expandable settings in future.
* Using DDD as a new way to design grids

Grids are determined in a folder `app/grids`. For example:
```ruby
class ProductsGrid < MightyGrid::Base
  scope { Product }
end
```
and you can create instance of class `UsersGrid` in your controller:
```ruby
# Before
def index
  init_grid(User, options)
end
# After
def index
  UsersGrid.new(params, options)
end
```